Seismic exploration is one of the most powerful techniques for investigating the configuration of the subterranean formations beneath the earth's surface. The typical end product of a seismic survey is a map, termed a "seismic depth section," indicating the thickness and orientation of the various strata underlying that portion of the earth's surface from which the survey was conducted. By correlating the seismic depth section with other geologic information, such as data concerning surface outcroppings of various strata, wellbore cuttings and logs, and previous seismic sections, detailed information concerning the outermost several kilometers of the earth's crust can be developed. The predominant use of seismic exploration is in the search for subsurface structures favorable to the existence of oil and gas reservoirs.
Seismic reflection surveys, the most common type of seismic survey, are performed by initiating a shock wave at the earth's surface and monitoring at a plurality of surface locations the reflections of this disturbance from the underlying subterranean formations. These reflections occur from regions where there is a change in the reflectivity of the earth, generally the interfaces between adjacent strata. The devices used to monitor the reflections are termed geophones. The signal recorded by each geophone represents, as a function of time, the amplitude of the reflections detected by that geophona. This signal is commonly referred to as a "trace". The lateral distance along the earth's surface between the seismic source and a particular geophone is known as the "offset" of that geophone.
In performing a seismic survey, a large number of geophones, as many as one thousand or even more, are positioned along the line of the survey. Accordingly, for each shot numerous traces are obtained. At the time each trace is recorded, it is uniquely designated on the basis of source and detector position. In this manner, every trace is uniquely identified relative to all other traces. This information is later utilized in processing and displaying the traces.
The traces obtained in performing the survey must be processed prior to final display and analysis to compensate for various factors which impede utilization of the original traces. One of the most troublesome of the processing steps involves compensating for the nonhyperbolic distortion of the traces caused by the uppermost layer of the earth, typically 10-100 meters thick, termed the "low-velocity-layer" or "weathered layer" (hereinafter, the "LVL"). The velocity of seismic compressional waves (p-waves) through the LVL is typically in the range of 500-1000 meters per second, while p-wave velocities in the strata below the LVL are typically in excess of 1500 meters per second. The velocity of seismic shear waves (s-waves) is also less in the LVL than it is in the strata below the LVL. Because the LVL often varies greatly in thickness over relatively short horizontal distances, the transit time of a seismic wave through the LVL can vary significantly over the line of a seismic survey. If not corrected for, this variation can significantly hamper subsequent data processing and interpretation and can result in erroneous calculations of the configuration and depth of the underlying strata. Because even small variations in the calculated orientation of rock strata can have a major impact on decisions regarding the probability of oil and gas being found at a certain subterranean location, it is important that aberrations caused by the LVL be removed with the greatest precision possible.
Conventional seismic data processing uses refraction statics to compensate for the effects of the LVL. In refraction statics, traveltime corrections are determined from first arrivals on offset data traces. These traveltime corrections are then applied to the data to approximate the reflection arrival times which would have been observed if all measurements had been made on a flat datum plane with no LVL present.
As will be well known to those skilled in the art, first arrival refraction analysis may be used to estimate refractor velocity (i.e., the seismic velocity in the stratigraphic layer immediately below the base of the LVL) and LVL thickness, if the seismic velocity in the LVL ("LVL velocity") is inferred. Refraction data alone are insufficient to uniquely determine the refractor velocity, LVL velocity, and LVL thickness. Various well-known methods exist for estimating the LVL velocity, including the use of check shot surveys and/or well logs, standard velocity analysis of small offset reflections, and interpretation of all available geologic data. In some cases, it may be possible to estimate the LVL velocity as some function of the refractor velocity.
Once the LVL velocity and depth have been estimated, approximate vertical propagation traveltimes through the LVL are calculated. Each seismic data trace is then time advanced by this amount, simulating the effect of having sources and receivers located at the base of the LVL. Then, appropriate vertical time shifts may be computed, using a "replacement" velocity, which will redatum the sources and receivers to a horizontal datum. Each trace is time delayed by this amount. The replacement velocity is usually chosen to be the refractor velocity so as to make the near-surface region appear homogeneous. In many situations, this methodology yields acceptable results; however, seismic velocity replacement with static shifts assumes that only vertical wave propagation occurred through the LVL and that no ray-bending occurred at the LVL base. These assumptions are usually inadequate in the presence of complicated, near-surface geology, such as may be found in fold and thrust belt regions, because of the severe raypath bending that occurs at the LVL base and the unusually thick LVLs present. In these areas, refraction statics do not adequately compensate for the nonhyperbolic data distortion caused by the LVL.
It would be desirable to provide a method for removing the LVL-induced nonhyperbolic distortion of land seismic data which overcomes the above-identified problems of the traditional refraction statics approach. The present invention provides such a method.